Porous honeycomb structure

ABSTRACT

A porous honeycomb structure includes: a plurality of partition walls containing cordierite as a main component and constituted of a porous ceramic having a porosity of 55 to 75% and an average pore diameter of 15 to 35 μm, wherein the partition walls have a pore distribution represented by the following condition formula ( 1 ): 
 
 Lr &gt;0.3× P /100+0.91  (1), 
         “in the above condition formula ( 1 ), Lr means an average developed length ratio, and P means a porosity obtained from a total pore volume measured by a mercury press-in type porosimeter, assuming that a true specific gravity of cordierite is 2.52 g/cc.”The porous honeycomb structure is capable of effectively achieving raising of a trapping efficiency of soot or the like, lowering of a pressure loss, improving a purifying performance by effective use of a catalyst, and lengthening a trapping time, and is additionally capable of improving a dissolved loss limit at the time of filter regeneration, and an isostatic strength.

TECHNICAL FIELD

The present invention relates to a porous honeycomb structure. Thepresent invention relates particularly to a porous honeycomb structureapplicable as a filter for purifying an exhaust gas, which is capable ofeffectively lengthening a trapping time, raising a trapping efficiency,and lowering a pressure loss, when a pore distribution of partitionwalls is controlled and which is further capable of preventing adissolved loss at the time of filter regeneration or improving anisostatic strength, and as a catalyst carrier which is capable ofeffectively improving a purifying performance and lowering a pressureloss and which is superior in isostatic strength.

BACKGROUND ART

In recent years, influences of particulate matters or NOx exhausted froman engine for an automobile, especially a diesel engine or the like ontoenvironments have been brought into the public eye, and various uses ofporous honeycomb structures as important means for removing these toxicsubstances have been studied.

For example, in a honeycomb structure including a plurality ofthrough-holes partitioned by porous partition walls, a honeycomb filterhas been developed including a structure in which the through-holes areplugged in different positions on opposite end faces including thethrough-holes opened therein. An exhaust gas is allowed to flow in eachthrough-hole opened in one end face, and forcibly passed through thepartition walls in the honeycomb structure to trap and remove theparticulate matters in the exhaust gas. As new attempts to increase acatalyst support amount and to thereby improve a purifying performance,a catalyst body of a honeycomb structure has also been developedincluding the partition walls which are all formed into porous bodieshaving high porosities and which support a catalyst for decomposing HCor NOx.

Additionally, in the porous honeycomb structure, a high trappingefficiency has been naturally required in the application as the filter.When a certain or more amount of soot is deposited on pores opened inthe surfaces of the partition walls, the pressure loss rapidlyincreases. Therefore, usually after the use for a certain time, aregeneration step is performed to burn the filter at a high temperatureand to thereby burn up the soot. However, when this regeneration step isperformed with a high frequency, degradation of the porous honeycombstructure is promoted. Therefore, a trapping time has been required tobe lengthened to decrease the number of regeneration steps.

Furthermore, in the application as the filter, since the poroushoneycomb structure is exposed at a remarkably high temperature at thetime of the filter regeneration, the whole filter preferably has acertain or more thermal capacity in order to prevent a dissolved loss ofthe partition wall by the filter regeneration. When the soot is burnt, amaximum soot amount (soot limited regeneration amount) that does notcause the dissolved loss of the partition wall during the regenerationof the filter is required to be large.

On the other hand, in the application as the catalyst carrier, in recentyears, there has been a demand for the increasing of a catalyst supportamount for further improvement of an exhaust gas purifying performance,and attempts have been made to support the catalyst on the honeycombstructure whose porosity has been raised.

Moreover, in any application, there has been a demand for the reductionof the pressure loss in the porous honeycomb structure installed in acombustion engine such as a diesel engine strongly required to havereduced fuel consumption and increased output. Furthermore, since theporous honeycomb structure is disposed in the vicinity of the enginecontinued to be largely vibrated in any application, the structure needsto be firmly grasped in a metal case, and the whole honeycomb structureis required to have a high isostatic strength.

To meet these demands, a honeycomb structure or the like has heretoforebeen disclosed in which the pore distribution of the partition walls iscontrolled in various ranges.

For example, in Japanese Patent No. 2726616, a honeycomb structure hasbeen described in which a specific surface area (Mm²/g) of the poreopened in the surface of the partition wall and a surface roughness(Nμm) in the filter surface are controlled in 1000M+85N≧530.

However, the honeycomb structure is manufactured using ceramic rawmaterials such as silica and talc whose particle diameters have beencontrolled, and the porosity is about 60% at maximum (Japanese PatentNo. 2726616).

Moreover, in the honeycomb structure, the distribution of the poresinside the partition walls, except a ratio of the pores opened in thesurface of the partition wall with respect to the porosity of the wholepartition wall, is not considered, and demands for the lengthening ofthe trapping time, the raising of the trapping efficiency, the improvingof the purifying performance, and the lowering of the pressure loss havenot been sufficiently satisfied.

On the other hand, a cordierite honeycomb structure (Japanese PatentApplication Laid-Open No. 9-77573) has been proposed in which an organicfoaming agent and carbon are added as pore formers to a cordierite rawmaterial for use, and the number of 5 to 40 μm small holes is set to befive to 40 times that of 40 to 100 μm large holes among the pores in thesurface of the partition wall.

However, also in this honeycomb structure, the ratio of the pores openedin the partition wall surface with respect to the porosity of the wholepartition wall has not been considered. The honeycomb structure has beenmanufactured using an organic foaming agent which originally containsdense particles and which is hollowed when heated as a pore formermaterial. Therefore, there is little organic foaming agent resulting inthe opened surface of the partition wall just after extrusion molding.When the binder gels by heat in the subsequent drying step or the like,and the formed article is hardened, the surface of the partition wall isnot largely expanded in such a manner as to be burst even by the foamingat a comparatively low temperature of 100° C. or less, and the number ofpores opened in the partition wall surface has been small in the presentsituation. As a result, in the honeycomb structure, the poredistribution of the partition wall in a thickness direction has adeviation, and the demands for the raising of the trapping efficiency,the lengthening of the trapping time, and the lowering of the pressureloss have not been sufficiently satisfied. Since the amount of thecatalyst supported inside the partition walls is very large because ofthe deviation of the pore distribution, an effective use ratio actuallycontributing to a purifying reaction is small, and a sufficientpurifying performance has not been obtained. Furthermore, when theporosity of the whole partition wall is further raised in order to solvethe problem, there has been a problem that a local dissolved loss on thepartition wall at the time of the filter regeneration is caused by thedecrease of the isostatic strength or the thermal capacity.

Moreover, even in the honeycomb structure manufactured using non-foamingpore formers such as PMMA and PET, the number of pores opened in thepartition wall surfaces has been small in the present situation asdescribed above. The demands for the effective raising of the trappingefficiency, the lengthening of the trapping time, the improving of thepurifying performance, and the lowering of the pressure loss are notsufficiently satisfied. When the porosity of the whole partition wall isfurther raised, there has been a problem that the local dissolved lossof the partition wall at the time of the filter regeneration is causedby the decrease of the isostatic strength or the thermal capacity.

DISCLOSURE OF THE INVENTION

The present invention has been developed in consideration of theabove-described problems, and an object thereof is to provide a poroushoneycomb structure which is capable of effectively raising a trappingefficiency of soot or the like, improving a purifying performance byeffective use of a catalyst, lowering a pressure loss, and lengthening atrapping time and which is further capable of improving a dissolved losslimit at the time of filter regeneration and an isostatic strength.

Moreover, as a result of intensive researches for achieving the objectby the present inventor, findings have been obtained that when ahoneycomb structure is manufactured by use of clay containing a foamedfoaming resin added thereto, the honeycomb structure is obtained havinga high porosity, including a large number of pores opened/formed inpartition wall surfaces, and having a pore distribution in which apartition wall developed length ratio is not less than a specific valuewith respect to the whole porosity. It has been found that according tothe honeycomb structure, the above-described conventional problems canbe solved, and the present invention has been completed.

That is, according to the present invention, there is provided a poroushoneycomb structure comprising: a plurality of partition wallscontaining cordierite as a main component and constituted of a porousceramic having a porosity of 55 to 75% and an average pore diameter of15 to 35 μm, wherein pores of the partition walls have a poredistribution represented by the following condition formula (1)(hereinafter sometimes referred to simply as “Condition Formula (1)”).Lr>0.3×P/100+0.91  (1)

“In Condition Formula (1), Lr means an average developed length ratioobtained by the following equation (2) (hereinafter referred to simplyas “Equation (2)”, and P means a porosity obtained from a total porevolume measured by a mercury press-in type porosimeter, assuming that atrue specific gravity of cordierite is 2.52 g/cc.”Lr=Lo/4  (2)

“In Equation (2), Lo means an average developed length (an average valueof lengths including the surfaces of the pores opened in the partitionwall surfaces) obtained when using a surface roughness measuringinstrument and checking optional ten places on the partition wallsurfaces every 4 mm (straight line length ignoring presence of the poresopened in the partition wall surfaces) along the partition wall surfaceswith a stylus, and Lr means the average developed length ratio.”

Moreover, in the present invention, further the pores of the partitionwalls preferably have a tomographic pore distribution represented by thefollowing condition formula (3) (hereinafter sometimes referred tosimply as “Condition Formula (3)”) in a partition wall thicknessdirection.X<−33×P/100+28  (3)

“In Condition Formula (3), X denotes an average value of a primarycomponent amplitude spectrum (F) and a secondary component amplitudespectrum (S) obtained from the following equations (4) and (5)(hereinafter sometimes referred to as “Equation (4)”, “Equation (5)”,respectively), and P means a porosity obtained in the same manner as inCondition Formula (1).”F={square root}{square root over (X _(SRe) (1) ² +X _(Slm) (1) ² )}  (4)

“In Equation (4), F denotes the primary component amplitude spectrumassuming k=1 in the following conversion equation (6), and X_(SRe)(1)and X_(Slm)(1) denote a real part and an imaginary part, respectively,assuming k=1 in Conversion Equation (6).”S={square root}{square root over (X _(SRe) (2) ² +X _(Slm) (2) ² )}  (5)

“In Equation (5), S denotes the secondary component amplitude spectrumassuming k=2 in Conversion Equation (6), and X_(SRe)(2) and X_(Slm)(2)denote a real part and an imaginary part, respectively, assuming k=2 inConversion Equation (6).” $\begin{matrix}{{X_{s}(k)} = {\sum\limits_{n = 0}^{255}{{x(n)}\left( {{\cos\quad{\frac{2\pi\quad k}{256} \cdot n}} - {{jsin}{\frac{2\pi\quad k}{256} \cdot n}}} \right)}}} & (6)\end{matrix}$

“In Conversion Equation (6), X_(S)(k) denotes a discrete Fouriertransform, k denotes a degree, n denotes an integer of 0 to 255indicating a divided position, when a partition wall section is dividedinto 256 in order in a thickness direction from a partition walloutermost surface portion (n=0), and X(n) denotes an area ratio occupiedby a pore portion in a partition wall section region to the dividedposition of n to n+1.”

In the present invention, a thickness of the partition wall ispreferably set to 350 μm or less. A coefficient of thermal expansion at40 to 800° C. can be set to 1.0×10⁻⁶/° C. or less with respect to thewhole honeycomb structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a partially sectional view of a partition wall,showing a method of measuring a tomographic porosity distribution;

FIG. 2 is a graph showing an example of the tomographic porositydistribution obtained by a method shown in FIG. 1;

FIG. 3 is a graph showing a result of Fourier transform of thetomographic porosity distribution shown in FIG. 2 with each series;

FIG. 4 is a graph showing results of examples and comparative exampleswith respect to a relation between a porosity and a developed lengthratio;

FIG. 5 is a graph showing results of the respective examples andcomparative examples with respect to a relation between the porosity andan average value (X) of a primary component amplitude spectrum (S) and asecondary component amplitude spectrum (F); and

FIG. 6 is a graph showing results of the respective examples andcomparative examples with respect to a relation between the averagevalue (X) of the primary component amplitude spectrum (S) and thesecondary component amplitude spectrum (F), and a soot trapping pressureloss.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, a porous honeycomb structure of the presentinvention contains cordierite as a main component, includes pores havinga specific porosity and average pore diameter, and further has aporosity distribution in which a developed length ratio and porosity ofeach partition wall surface satisfy a specific condition formula. Anembodiment of the present invention will be described hereinafterconcretely.

In the present invention, cordierite which is the main component of thepartition wall may be either oriented, non-oriented, α-crystalline,β-crystalline or the like. As components other than cordierite, forexample, mullite, zircon, aluminum titanate, clay bond silicon carbide,zirconia, spinel, indialite, sapphirine, corundum, titania or the likemay be contained alone, or two or more of them may also be contained.

Moreover, in the present invention, the partition walls of the honeycombstructure are constituted of a high-porosity ceramic having a porosityof 55 to 75%, more preferably 65 to 75%.

When the porosity is less than 55%, a transmission resistance in thepartition wall against the exhaust gas is excessively large, and it istherefore difficult to lower a pressure loss to such an extent that anypractical trouble is not caused even when a pore distribution iscontrolled as described later. It is also difficult to obtain a desiredcatalyst support amount. On the other hand, when the porosity exceeds75%, the isostatic strength lowers, and the structure easily breaks whenheld into a case.

Moreover, in the present invention, the pores existing in the partitionwalls of the porous ceramic have an average pore diameter of 15 to 35μm.

When the average pore diameter is less than 15 μm, a trapping efficiencyrises, but an initial pressure loss increases. On the other hand, whenthe average pore diameter exceeds 35 μm, and even when the poredistribution is controlled as described later, it becomes difficult tosecure a practically required trapping efficiency. Since soot is easilydeposited inside the partition walls, and the soot is easily unburnedinside the partition walls at the time of regeneration. Therefore, evenafter the regeneration, a soot deposited pressure loss does not lower insome case.

The honeycomb structure of the present invention further has a poredistribution in which the developed length ratio of each partition wallsurface is not less than a specific numeric value with respect to theporosity of the whole partition wall, and concretely has a poredistribution which satisfies a condition of Condition Formula (1).

Accordingly, since a substantially useful soot deposited allowablevolume increases in all the pores, the trapping efficiency iseffectively raised, and a trapping time can be lengthened. Since a ratioof pores opened in the partition wall surfaces with respect to theporosity of the whole partition wall is large, the pressure loss iseffectively lowered, or the catalyst support amount or an effective useratio of the supported catalyst is increased, and accordingly apurifying performance can be effectively improved. As apparent from theabove, even when the porosity of the whole partition wall is lowered toa certain degree, it is possible to obtain sufficient characteristicswith respect to the trapping efficiency, trapping time, purifyingperformance, and pressure loss, and additionally the isostatic strengthor a soot limited regeneration amount can be increased.

The partition walls in the porous honeycomb structure of the presentinvention further have a pore distribution in which a tomographic poredistribution in a thickness direction is represented by ConditionFormula (3). This is preferable in that the characteristics required forthe honeycomb structure, such as the partition wall dissolved loss atthe time of filter regeneration, the isostatic strength, the trappingefficiency, the purifying performance, the pressure loss, and thetrapping time, can be generally improved further.

That is, the honeycomb structure satisfying the condition of ConditionFormula (3) has little fluctuation of a tomographic porositydistribution, and the pores existing in the partition walls areuniformly distributed without deviating inside the partition walls.Therefore, since the soot is hardly deposited inside the partitionwalls, and the amount of the soot remaining after the filterregeneration is small, the soot deposited pressure loss can further bereduced. Moreover, the honeycomb structure satisfying the condition ofCondition Formula (3) does not lower the isostatic strength or the sootlimited regeneration amount because of a decrease of the thermalcapacity locally in each portion of the partition wall. Therefore, whilesatisfying these characteristics, the raising of the trappingefficiency, the lengthening of the trapping time, and the lowering ofthe pressure loss can be attained at higher levels. Similarly, since thecatalyst support amount or the effective use ratio of the supportedcatalyst is further increased, the isostatic strength is satisfied,whereas a higher purifying performance can be achieved.

Here, the tomographic porosity distribution in the partition wallthickness direction shown in Conversion Equation (6) can be measured bya scanning electronic microscope (SEM). Concretely, first a partitionwall section of the honeycomb structure is observed at a magnificationof 100 times, image data is binarized by image analysis software, ablack portion is regarded as a pore portion, and a white portion isregarded as a honeycomb substrate portion. Moreover, as shown in FIG. 1,the section is divided into 256 in the partition wall thicknessdirection by a width of 1 mm in a partition wall length direction, and aratio of an area occupied by the pore portions in each dividedtomographic region is calculated. FIG. 2 shows an example of atomographic porosity distribution X(n) obtained in this manner, and FIG.3 shows results of Fourier transform of the tomographic porositydistribution X(n) shown in FIG. 2 with each series.

In the present invention, there is not any special restriction onrespects other than various conditions of the pores disposed in thepartition walls including the partition wall thickness, cell shape, cellpitch or the like of the honeycomb structure, and the respects may bepreferably designed in accordance with applications or the like.Moreover, to impart a desired filter function, a structure is preferablein which a plurality of through-holes opened in an exhaust gasinflow-side end face and an exhaust gas exhaust-side end face areplugged in different positions on the opposite end faces.

Moreover, a catalyst can be supported on the porous partition walls ofthe honeycomb structure to form a catalyst body. In the use as thecatalyst carrier, the cell density is preferably in a range of 6 to 1500cells/inch² (0.9 to 233 cells/cm²), and the thickness of the partitionwall is preferably in a range of 50 to 2000 μm (about 2 to 79 mil). Anaxial direction (exhaust gas flow direction) length in the use as thecatalyst carrier is usually 60 to 300 mm, preferably 100 to 250 mm.

Furthermore, an adsorptive layer may also be disposed on the catalystcarrier, and a layer having a large specific surface area and containingalumina or zeolite as a main component is usually preferably used.Zeolite may be either natural or synthetic, and a type thereof is notespecially restricted, but an Si/Al ratio of 40 or more is forpreferable use from standpoints of heat resistance, durability, andhydrophobic property. Concretely, ZSM-5, USY, β-zeolite, silicalite,metallo silicate, and the like are preferably usable.

Additionally, catalyst components may be supported directly on thehoneycomb structure, and may also be supported on the adsorptive layer.

In the present invention, the whole partition wall has a uniform poredistribution, and the porosity can also be lowered to a certain degreeas described above. Therefore, even when the partition walls are furtherthinned, the dissolved losses of the partition walls are prevented, anda desired isostatic strength can be secured. Therefore, when thepartition wall thickness is set to 350 μm or less, a porous honeycombstructure having less pressure loss or higher catalyst effective useratio can be constituted.

Further in the honeycomb structure of the present invention, when afoamed foaming resin that does not inhibit a cordierite reaction is usedas means for forming pores in the vicinity of the partition wallsurfaces, a coefficient of thermal expansion at 40 to 800° C. can be setto 1.0×10⁻⁶/° C. or less, and a resistance to thermal shock at the timeof use at a high temperature can be improved.

In the present invention, as a method of attaining the above-describedpore distribution, a manufacturing method is preferable in which thefoamed foaming resin such as acrylic microcapsule is added as a poreformer to a cordierite raw material, and kneaded to form clay for use.

When hollow clay containing the foamed foaming resin added thereto isused from the first, the foamed foaming resin released from a pressingforce expands in the partition walls immediately after extrusion moldingof the clay, the resin existing in the vicinity of the partition wallsurfaces protrudes from the partition wall surfaces and expands, finallyan outer wall bursts, and a large number of pores opened in thepartition wall surfaces are formed. The state before the drying isalready similar to a state in which a large number of pores are formed.Therefore, even when the binder gels and accordingly a honeycomb formedarticle hardens in the subsequent drying step or the like, this does notdecrease the number of pores opened in the partition wall surfaces.

Additionally, in the present invention, a method may also be combined inwhich particle diameters of ceramic raw materials such as talc andsilica are controlled to control the porosity and the average porediameter of the obtained filter. However, when the particle diametersfor use are excessively large, it is difficult to obtain a desired poredistribution even with combined use of the foamed foaming resin.Therefore, talc having a particle diameter of 50 μm or less, or silicahaving a particle diameter of 100 μm or less is preferably used.

Furthermore, in the present invention, other materials such as carbon,flour, starch, phenol resin, polymethyl methacrylate, polyethylene, andpolyethylene terephthalate may also be contained as the pore former.Above all, carbon such as graphite is preferable because carbon burns ina temperature range different from that in which the binder or thefoaming resin is oxidized/decomposed to generate heat, and thereforecalcining shortage is not easily caused.

Moreover, when the foamed foaming resin is used alone as the poreformer, preferably 0.5 to 5 parts by mass, more preferably 1 to 3.5parts by mass of the foamed foaming resin is contained with respect to100 parts by mass of the cordierite raw material.

When the content of the foamed foaming resin is less than 0.5 parts bymass, the porosity is less than 55%, and the pressure loss of theobtained honeycomb structure increases. On the other hand, when thecontent of the foamed foaming resin exceeds 5 parts by mass, theporosity is larger than 75%, and the isostatic strength and the thermalcapacity of the obtained honeycomb structure lower, and the structurehardly withstands practical use.

Furthermore, when carbon such as graphite is used together with thefoamed foaming resin, from similar standpoints, 5 to 25 parts by weightof carbon, and 0.5 to 3 parts by weight of the foamed foaming resin arepreferably contained with respect to 100 parts by weight of thecordierite raw material, and 5 to 15 parts by weight of carbon and 1 to3 parts by weight of the foamed foaming resin are more preferablycontained.

In the present invention, usually, as other additives, hydroxypropylmethyl cellulose, methyl cellulose, hydroxyethyl cellulose, carboxylmethyl cellulose, binder such as polyvinyl alcohol, ethylene glycol,dextrin, fatty acid soap, dispersant such as polyalcohol and the likemay be preferably contained. It is to be noted that in the presentinvention, even when the binder gels and accordingly the formed articlehardens in the drying step, any deviation is not generated in the poredistribution as described above.

The present invention will be described hereinafter more concretely inaccordance with examples, but the present invention is not limited tothese examples. It is to be noted that honeycomb structures obtained inexamples and comparative examples were evaluated in the followingmethod.

1. Evaluation Method

(1) Average Pore Diameter of Pores

The diameter was measured by a mercury press-in type porosimetermanufactured by Micromeritics Co., Ltd.

(2) Porosity

A total pore volume was measured by the mercury press-in typeporosimeter manufactured by Micromeritics Co., Ltd., a true specificgravity of cordierite was assumed as 2.52 g/cc, and a porosity wascalculated from the total pore volume.

(3) Average Developed Length Ratio

A surface roughness measuring instrument FTS-S4C manufactured by TaylorHobson Co., Ltd. was used, and a 2 μm R stylus tip was brought intocontact with the partition wall surface of each of optionally selectedten places by 4 mm in a straight line length (length ignoring thepresence of the opened pores) of the partition wall surface along thepartition wall surface. A developed length (length in which the innersurface of an opened pore is considered) corresponding to the straightline length was measured. Next, an average value (Lo) of developedlengths on the partition wall surfaces of ten places was obtained, andthe average developed length (Lo) was divided by 4 which was thestraight line length to obtain an average developed length ratio (Lr).The average developed length ratio was divided by the porosity of thewhole partition wall obtained in the above-described method to obtainthe average developed length ratio per unit porosity.

(4) Uniformity of Tomographic Porosity (Average Value (X) of PrimaryComponent Amplitude Spectrum (S) and Secondary Component AmplitudeSpectrum (F))

As shown in FIG. 1, the partition wall section of a honeycomb structurewas photographed by SEM in a range of 1 mm in a partition wall lengthdirection, image data was binarized by image analysis software, a blackportion was regarded as a pore portion, and a white portion was regardedas a honeycomb substrate portion. Thereafter, a 300 μm partition wallwas divided into 256 in a thickness direction, and an area ratio of thepore portion in each divided region (n to n+1) was calculated in orderfrom the partition wall surface as shown in FIG. 2. Subsequently, achange of the area ratio of the pore portion in the partition wallthickness direction was Fourier-transformed to obtain an amplitudespectrum of each wave as a wave having a frequency (n)=1, 2, and aprimary component amplitude spectrum (S), and a secondary componentamplitude spectrum (F) were obtained, respectively. Finally, an averagevalue (X) of these numeric values was obtained to evaluate uniformity ofa tomographic porosity.

(5) Trapping Efficiency

An exhaust gas in which soot was generated by a soot generator waspassed through honeycomb structures obtained in examples and comparativeexamples for a certain time (two minutes). After the passing through afilter, the soot contained in the exhaust gas was trapped by filterpaper to measure a weight (W¹) of the soot. At the same time, theexample containing the soot generated therein was trapped by the filterpaper without being passed through the filter to measure a weight (W²)of the soot. Subsequently, the respective obtained weights (W¹) (W²)were substituted into the following equation (7) to obtained a trappingefficiency.(W ² −W ¹)/(W ²)×100  (7)

(6) Soot Trapping Pressure Loss

First, rings each having an inner diameter of φ130 mm werepressure-welded to opposite end faces of each of honeycomb structuresobtained in the respective examples and comparative examples, and sootgenerated by a soot generator was passed into a range of φ130 mm of thehoneycomb structure via the ring to trap 10 g of soot. Subsequently, ina state in which the soot was trapped by the honeycomb structure, 2.27Nm³/min of air was passed, and a pressure difference before/after afilter was measured to evaluate a pressure loss in the state in whichthe soot was trapped.

(7) Exhaust Gas Purifying Efficiency

A catalyst body in which a catalyst was supported by each of thehoneycomb structures obtained in the respective examples and comparativeexamples was canned in a metal case, a 5 liter diesel engine was used,and a purifying efficiency of HC was evaluated. An exhaust gas from theengine was passed through the catalyst body of the honeycomb structure,and an HC concentration B1 in the exhaust gas before introduction intothe catalyst body, and an HC concentration B2 in the exhaust gas afterthe passing through the catalyst body were measured to calculate100×(B1−B2)/B1.

(Evaluation Results)

In each of honeycomb structures of Examples 1 to 10 in which 2 to 3.5parts by mass of a foamed foaming resin was added as a pore former asshown in Tables 2, 3, an average pore diameter of partition walls was16.0 to 34.6 μm, and a porosity was 55.5 to 72.0%. In any example, apore distribution of the obtained honeycomb structure satisfiedconditions of Condition Formulas (1) and (3) as shown in FIGS. 4 and 5.Even in the honeycomb structures of Examples 1 and 7 in whichcomparatively coarse talc and silica raw materials were used, theconditions of these relation formulas were satisfied, and this wassupposedly because the foamed foaming resin expanded in the partitionwalls immediately after the forming.

On the other hand, as pore formers, 2 parts by mass of the foamedfoaming resin constituted of a graphite methyl methacrylate, 4 parts bymass of hydroxypropyl methyl cellulose, 0.5 parts by mass of lauricpotash soap, and 30 parts by mass of water were projected and kneaded toobtain a plastic property. This plastic raw material was formed intocylindrical clay by a vacuum soil kneader, and projected into anextruder to form a honeycomb shape.

Subsequently, the obtained formed article was absolutely dried by hotair drying after dielectric drying, and opposite end faces thereof werecut into predetermined dimensions.

Subsequently, through-holes in a dried honeycomb article werealternately plugged in the opposite end faces in which the through-holesare opened with slurry constituted of a cordierite raw material having asimilar composition.

Finally, the article was calcined at 1420° C. for four hours to obtain ahoneycomb structure (honeycomb filter) having a size: φ144 mm×L 152 mm,a partition wall thickness: 300 um, and the number of cells: 300cells/inch².

EXAMPLES 2 TO 10, AND COMPARATIVE EXAMPLES 1 TO 8

Honeycomb structures (honeycomb filters) were obtained in the samemanner as in Example 1 except that cordierite raw materials and poreformers having compositions shown in Tables 1 and 2 were used in Example1.

(Evaluation Results)

In each of honeycomb structures of Examples 1 to 10 in which 2 to 3.5parts by mass of a foamed foaming resin was added as a pore former asshown in Tables 2, 3, an average pore diameter of partition walls was16.0 to 34.6 μm, and a porosity was 55.5 to 72.0%. In any example, apore distribution of the obtained honeycomb structure satisfiedconditions of Condition Formulas (1) and (3) as shown in FIGS. 4 and 5.Even in the honeycomb structures of Examples 1 and 7 in whichcomparatively coarse talc and silica raw materials were used, theconditions of these relation formulas were satisfied, and this wassupposedly because the foamed foaming resin expanded in the partitionwalls immediately after the forming.

On the other hand, in honeycomb structures of Comparative Examples 1 to4, 6 manufactured using graphite, PET, and PMMA as the pore formers,pores were unevenly distributed inside, and any of the conditions ofCondition Formulas (1) and (3) was not satisfied as shown in FIGS. 4 and5.

Therefore, as shown in FIG. 5, in the porous honeycomb structures of therespective examples, as compared with the honeycomb structures of therespective comparative examples having almost the same degree ofporosities, the average value (X) of the primary component amplitudespectrum and the secondary component amplitude spectrum was small. Whenthe characteristics of the honeycomb structures having the equalporosity were compared, as shown in Table 3, the honeycomb structure ofeach example had a higher trapping efficiency and a lower trappingpressure loss as compared with those of the honeycomb structure of eachcomparative example. As shown in FIG. 6, all through the honeycombstructures, a tendency has been recognized that the smaller the averagevalue (X) of the primary and secondary component amplitude spectrums is,a soot trapping pressure loss decreases.

On the other hand, in the honeycomb structure of Comparative Example 5in which a very coarse silica raw material was used, since some ofsilica particles appeared on the partition wall surfaces to form pores,the condition of Condition Formula (1) was satisfied, but the averagepore diameter was 35 μm or more, and the condition of Condition Formula(3) was not satisfied. Therefore, in the honeycomb structure, thetrapping efficiency was 65% and very low, and the soot trapping pressureloss was very large.

Moreover, in the honeycomb structure of

Comparative Example 7 manufactured using a non-foamed foaming resinfoaming at 80° C. or more, either of the conditions of ConditionFormulas (1) and (3) was not satisfied, the porosity was 44.4% and verylow, and therefore the soot trapping pressure loss became very large.This was supposedly because the binder gelled and accordingly the formedarticle hardened in the drying step, and the non-foamed foaming resinwas inhibited from being expanded in the formed article.

Furthermore, in the honeycomb structure of Comparative Example 8 inwhich 10 parts by weight of graphite was used together with 3.5 parts byweight of the foamed foaming resin, the porosity was 75% or more, theisostatic strength was very small, and therefore it was not possible toset the structure onto a jig for measurement of the soot trappingpressure loss. TABLE 1 Average particle diameter (μm) Talc A 20 Talc B45 Kaolin 10 Alumina 5 Aluminum hydroxide 3 Molten silica A 25 Moltensilica B 40 Silica A 5 Silica B 130 Graphite 40 Foamed foaming resin 50Non-foamed foaming resin 15 PMMA 55 PET 50

TABLE 2 Pore former blend ratio (wt %) Cordierite raw material blendratio (wt %) Foamed Non-foamed Aluminum Molten foaming foaming TalcKaolin Alumina hydroxide Silica silica Graphite resin resin PMMA PETExample 1 B:40 20 14 16 B:10 2 Example 2 B:40 20 14 16 A:10 2 Example 3B:40 20 14 16 A:10 10 2 Example 4 B:40 20 14 16 A:10 10 2 Example 5 B:4020 14 16 A:10 2.5 Example 6 A:39 19 14.5 16.5 B:11 2.5 Example 7 B:40 2014 16 A:10 10 2.5 Example 8 B:40 20 14 16 A:10 3 Example 9 B:40 20 14 16A:10 10 3 Example 10 B:40 20 14 16 A:10 3.5 Example 11 B:40 20 14 16B:10 2 Comparative B:40 20 14 16 B:10 15 10 Example 1 Comparative B:4020 14 16 A:10 20 10 2 Example 2 Comparative B:40 20 14 16 A:10 25 15 10 Example 3 Comparative B:40 20 14 16 A:10 25 25 10  Example 4 ComparativeB:40 20 14 16 B:10 20 10 5 Example 5 Comparative A:39 19 14.5 16.5 B:1120 10 5 Example 6 Comparative B:40 20 14 16 B:10 2 Example 7 ComparativeB:40 20 14 16 A:10 10 3.5 Example 8 Comparative B:40 20 14 16 B:10 15 10Example 9

TABLE 3 Partition Exhaust Coefficient wall surface Power spectrum X−Trapping gas Pore of thermal developed Lr− primary, (−33 × Trappingpressure purifying Porosity diameter expansion length ratio (0.3 ×porosity/ secondary porosity/ efficiency loss efficiency (%) (μm)(×10⁻⁶/° C.) Lr 100 + 0.91) average value X 100 + 28) (%) (kPa) (%)Example 1 55.5 25.4 0.4 1.09 0.01 7.6 −2.1 90 9.8 Example 2 55.7 20.40.4 1.09 0.01 7.2 −2.4 92 9.5 Example 3 59.6 24.6 0.6 1.12 0.03 6.1 −2.290 8.9 Example 4 60.5 16.0 0.4 1.10 0.01 4.1 −3.9 96 8.1 Example 5 59.525.2 0.6 1.13 0.04 4.0 −4.4 90 8.0 Example 6 59.1 34.6 0.7 1.14 0.05 7.7−0.8 85 9.4 Example 7 65.8 23.1 0.7 1.12 0.01 3.5 −2.8 91 7.1 Example 865.8 23.9 0.8 1.13 0.02 3.1 −3.2 90 6.8 Example 9 71.5 21.4 0.7 1.140.02 1.8 −2.6 93 6.2 Example 10 72.0 21.8 0.8 1.15 0.02 1.5 −2.7 92 5.9Example 11 the same as Example 1 93 Comparative 55.6 25.3 0.4 1.06 −0.0211.4 1.7 87 12.1 Example 1 Comparative 59.4 21.7 0.5 1.07 −0.02 9.1 0.788 11.1 Example 2 Comparative 65.1 22.4 0.7 1.09 −0.02 8.0 1.5 88 9.5Example 3 Comparative 70.9 22.6 0.8 1.09 −0.03 6.0 1.4 88 8.4 Example 4Comparative 58.0 39.2 1.1 1.13 0.05 13.7 4.8 65 12.1 Example 5Comparative 59.1 31.4 0.7 1.08 −0.01 11.8 3.3 79 11.4 Example 6Comparative 44.4 18.6 0.3 1.02 −0.02 14.1 0.8 97 13.1 Example 7Comparative 77.4 22.5 1.0 1.16 0.02 1.8 −0.7 Example 8 Comparative thesame as Comparative Example 1 85 Example 9* Comparative Example 8 had a remarkably small strength, and thereforewas not settable onto a measurement jig for measuring the trappingefficiency or the trapping pressure loss.

EXAMPLE 11

A honeycomb structure (catalyst carrier) having a size: φ229.0 mm×L152.0 mm, a partition wall thickness: 300 μm, and a cell density: 46.5cells/cm² was manufactured in the same manner as in Example 1 exceptthat a step of plugging through-holes was not performed in Example 1.

COMPARATIVE EXAMPLE 9

A honeycomb structure (catalyst carrier) having a size: φ229.0 mm×L152.0 mm, a partition wall thickness: 300 μm, and a cell density: 46.5cells/cm² was manufactured in the same manner as in Example 1 exceptthat a step of plugging through-holes was not performed, and acordierite raw material and a pore former having a composition similarto that of Comparative Example 1 as shown in Tables 1 and 2 were used inExample 1.

(Evaluation Result)

When 500 g of high specific surface area alumina and platinum-basedoxide catalyst were supported on an obtained honeycomb structure to forma catalyst body, an exhaust gas purifying efficiency was 93% and largein the honeycomb structure of Example 11. On the other hand, in thehoneycomb structure of Comparative Example 9, although 500 g of highspecific surface area alumina and platinum-based oxide catalyst weresimilarly supported to form a catalyst body, the exhaust gas purifyingefficiency was 85% and small as compared with the honeycomb structure ofExample 11.

Industrial Applicability

As described above, according to the present invention, there can beprovided a porous honeycomb structure suitable as exhaust gas purifyingmeans such as a diesel particulate filter and a catalyst carrier forpurifying an exhaust gas. More concretely, there can be provided ahoneycomb filter capable of effectively achieving raising of a trappingefficiency of soot or the like, lowering of a pressure loss, andlengthening of a trapping time, and additionally capable of improving adissolved loss limit at the time of filter regeneration and an isostaticstrength, and a catalyst carrier capable of effectively achievingimproving of a purifying performance by effective use of a catalyst, orlowering of a pressure loss, and superior in isostatic strength.

1-4. (canceled)
 5. A porous honeycomb structure comprising: a pluralityof partition walls containing cordierite as a main component andcomprising a porous ceramic having a porosity of 55 to 75% and anaverage pore diameter of 15 to 35 μm, characterized in that pores of thepartition walls have a pore distribution represented by the followingcondition formula (1):Lr>0.3×P/100+0.91  (1), “in the above condition formula (1), Lr means anaverage developed length ratio obtained by the following equation (2),and P means a porosity obtained from a total pore volume measured by amercury press-in type porosimeter, assuming that a true specific gravityof cordierite is 2.52 g/cc:”Lr=Lo/4  (2), “in the above equation (2), Lo means an average developedlength (an average value of lengths including the surfaces of the poresopened in the partition wall surfaces) obtained when using a surfaceroughness measuring instrument and checking optional ten places on thepartition wall surfaces every 4 mm (straight line length ignoringpresence of the pores opened in the partition wall surfaces) along thepartition wall surfaces with a stylus, and Lr means the averagedeveloped length ratio.”
 6. The porous honeycomb structure according toclaim 5, wherein the pores of the partition walls have a tomographicpore distribution represented by the following condition formula (3) ina partition wall thickness direction:X<−33×P/100+28  (3), “in the above condition formula (3), X denotes anaverage value of a primary component amplitude spectrum (F) and asecondary component amplitude spectrum (S) obtained from the followingequations (4) and (5), and P means a porosity obtained from the totalpore volume measured by the mercury press-in type porosimeter, assumingthat the true specific gravity of cordierite is 2.52 g/cc:”F={square root}{square root over (X _(SRe) (1) ² +X _(Slm) (1) ² )}  (4)“in the above equation (4), F denotes the primary component amplitudespectrum assuming k=1 in the following conversion equation (6), andX_(SRe)(1) and X_(Slm)(1) denote a real part and an imaginary part,respectively, assuming k=1 in the conversion equation (6):”S={square root}{square root over (X _(SRe) (2) ² +X _(Slm) (2) ² )}  (5)“in the equation (5), S denotes the secondary component amplitudespectrum assuming k=2 in the following conversion equation (6), andX_(SRe)(2) and X_(Slm)(2) denote a real part and an imaginary part,respectively, assuming k=2 in the conversion equation (6),”$\begin{matrix}{{X_{s}(k)} = {\sum\limits_{n = 0}^{255}{{x(n)}\left( {{\cos\quad{\frac{2\pi\quad k}{256} \cdot n}} - {{jsin}{\frac{2\pi\quad k}{256} \cdot n}}} \right)}}} & (6)\end{matrix}$ “in the conversion equation (6), X_(S)(k) denotes adiscrete Fourier transform, k denotes a degree, n denotes an integer of0 to 255 indicating a divided position, when a partition wall section isdivided into 256 in order in a thickness direction from a partition walloutermost surface portion (n=0), and X(n) denotes an area ratio occupiedby a pore portion in a partition wall section region to the dividedposition of n to n+1.”
 7. The porous honeycomb structure according toclaim 5, wherein a thickness of the partition wall is 350 μm or less. 8.The porous honeycomb structure according to claim 6, wherein a thicknessof the partition wall is 350 μm or less.
 9. The porous honeycombstructure according to claim 5, wherein a coefficient of thermalexpansion at 40 to 800° C. is 1.0×10⁻⁶/° C. or less.
 10. The poroushoneycomb structure according to claim 6, wherein a coefficient ofthermal expansion at 40 to 800° C. is 1.0×10⁻⁶/° C. or less.